LeetCode 669. Trim a Binary Search Tree

Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example:

Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

Solution:

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} L
 * @param {number} R
 * @return {TreeNode}
 */
var trimBST = function(root, L, R) {
    if(root == null)
        return null;
    if(root.val > R) {
        return trimBST(root.left, L ,R);
    }
    if(root.val < L) {
        return trimBST(root.right, L, R)
    }
    
    root.left = trimBST(root.left, L, R);
    root.right = trimBST(root.right, L , R);
    return root;
};